Parse/unparse absolute sheet name #65

Description

Extracted from #12

Some spreadsheet applications add dollar sign $ to sheet name to indicate it as an absolute reference. It is absolute by nature so ODFF allows making it optional.

• We can make it optional in regex so we can always parse copied formula from other sources. Then normalize it to our settings (default hardcoded is a reference without $)
• Add a config option to change the above default, and always unparse formula with $ before sheet name.

Spec

"OOo 2.0 always generates absolute Sheetnames if it's not the current sheet (i.e., '$' always prefixes a Sheetname). However, the OpenDocument 1.0 specification 8.3.1 clearly makes the "$" optional in front of a sheet name. "

This is a part of ODFF Variances and a nice to have.